West Nile threat: Minor bout might be your best protection - Chicago Tribune

An outbreak of West Nile virus in Dallas, Texas, prompted the county to declare a health emergency and launch aerial insecticide spraying after 200 cases and 10 deaths were reported this summer. But how much of a threat is the virus really -- and could a minor bout of the disease be the only way to protect yourself?

Anything that kills people, of course, is a concern. What's most worrying about West Nile is that it appears to be on the rise -- and that may not just lead to more West Nile cases. It also means its nastier cousins, such as dengue, chikungunya and tick-borne encephalitis, could flourish.

West Nile virus normally infects birds, and is carried between them by mosquitoes. In temperate regions, the number of infected birds rises steadily after mosquitoes become active in spring. By late summer, so many birds have been infected that the mosquito species which bite both birds and people occasionally carry the virus from a bird to a human.

Only one person in five infected develops (mostly flu-like) symptoms. About one person in 150 -- often elderly or with impaired immunity -- develops serious, sometimes fatal, illness.

In Europe, there are outbreaks in humans most years, but until recently they've remained sporadic. However, the virus is becoming endemic in parts of southern Europe. There were 93 reported cases in Europe last year, predominantly in Greece.

West Nile was not found in the western hemisphere until 1999, when it was accidentally introduced in New York, possibly from a bird imported from Israel. Within a few years, it had swept across the Americas and is now endemic, with annual outbreaks. This year, those outbreaks have started early: the U.S. Centers for Disease Control and Prevention in Atlanta, Ga., reports 693 cases in the United States so far -- the highest number seen by mid-August since 1999. Almost half of these are in Texas. In total, 26 people have died.

The surge could be a natural cycle: When the virus is prevalent, many birds are infected and become immune. As they do not transmit the virus, it becomes scarcer. A generation of birds then grows up with less immunity. They permit more spread of the virus, so it booms, and the cycle repeats.

But hot weather could have prompted the boom this year, boosting mosquito numbers and making the virus multiply faster. Moreover, "this year, with a mild winter, more mosquitoes survived and so we're seeing a bumper crop," says George DiFerdinando, at the University of Medicine and Dentistry of New Jersey in Piscataway.

"In theory, we know a vaccine is possible," says Ab Osterhaus, of Erasmus University in Rotterdam, the Netherlands. However, it may not be cost-effective to mass vaccinate, he says, given that relatively few of those infected will die. Meanwhile, what boosts West Nile boosts other insect-borne viruses. "There is more and more out there," says Osterhaus.

For now, all we can do is use insect repellent, drain stagnant water where mosquitoes breed, spray insecticide -- and be glad severe cases are rare in children. People who do catch the virus will become immune and might be the best protected as the boom continues.